Increasingly a number of manufacturing processes require the use of finely divided starting materials of, for example, particle size less than 5 microns, frequently of particle size less than 1 micron, and increasingly of particle size as small as 0.1 micron. This is particularly the case with processes for ceramics, where the use of such finely-divided raw materials makes it possible to produce articles having improved properties, such as improved strength, mechanical and thermal shock resistance, and of maximum or near maximum theoretical density after firing or sintering. The particle size distribution is also an increasingly important criterion, and particularly the requirement that all of the particles are of a size within a narrow range about the nominal value. In industrial practice the achievement of such uniformity of particle size is extremely difficult and considerably increases the cost of production.
For example, the manufacture of a ceramic part may require that the starting material be of average particle size 0.3 micron and maximum particle size 1.0 micron, such a small maximum size being necessary to permit, for example, the part to be superplastically forged. It is expected that the particle size distribution will have the typical bell-shape characteristic, with the majority of the material (e.g. about 70% by weight) of about the average size, while small portions (e.g. about 15% each) are oversize and undersize. Even though the material was milled to be of that average size, it is unlikely that as received by its ultimate user it is still in the same state of relatively uniform fine division, since with all particles, and particularly with such fine particles, agglomeration begins immediately the powder leaves the grinding mill, and continues during subsequent handling. Frequently the powders are pelletized to facilitate their transport and handling, and must subsequently be de-pelletized by grinding. The result is that the material is now nonuniform with at least a portion outside the specified range, and there is a high probability it includes a large number of big particles whose presence causes defects in the resultant sintered products. It is also important that the processing of the material, particularly the grinding, does not introduce any appreciable amount of contaminating particles, e.g. less than 0.1% by weight, and preferably less than 0.01% by weight.
Stone (carborundum) and colloid mills are known for use in paint pigment grinding and milling and consist essentially of two accurately shaped smooth stones working against each other, one of which is held stationary while the other is rotated at high speed (3600 to 5400 rpm) with a gap that is regarded by this industry as very small separating the two relatively movable surfaces. Thus, typically the spacing between the two faces is adjustable from positive contact to an appropriate distance, which with such mills is usually from a minimum of 25 micrometers to as much as 3,000 micrometers, but is usually of the order of 50-75 micrometers. In the typical stone mill a charge which is already mixed is fed through a truncated conical gap to the milling region, which has the shape of a flat annular ring, while in a colloid mill, which also requires an already mixed charge, the milling region has the shape of a truncated cone. The grinding of the pigment in its liquid vehicle is produced by the high shear rate smearing action that takes place between the parallel faces of the stones as the material is fed into the gap by gravity, or under pressure. A separation gap of 75 micrometers is said to produce a particle grind having an average particle size of 2-3 micrometers, although the particle size distribution is not given, and substantially larger particles are certainly present. Such mills are satisfactory for such purposes where the uniformity, particle size distribution, maximum particle size and the degree of contamination are relatively uncritical.